The present invention relates to semi-insulating silicon carbide single crystals, and in particular, relates to a method of forming high purity semi-insulating silicon carbide single crystal substrates that have intrinsic point defects and resulting deep level electronic states in an amount greater than the net concentration of any compensating shallow dopants (i.e., an amount greater than the uncompensated shallow dopants), and to maintain the semi-insulating quality of the silicon carbide substrate during additional process steps of device manufacture.
As set forth in the xe2x80x3680 patent and related applications, it has been discovered that semi-insulating silicon carbide can be produced without the use of vanadium as the dopant to create deep level states that produce the semi-insulating character.
Although vanadium can produce a semi-insulating silicon carbide crystal, its presence has been observed to create a back-gating effect; i.e., the trapped negative charge on the vanadium acts as a grown-in gate in devices in which a vanadium-doped crystal is used as the semi-insulating substrate. Thus, for a number of device considerations, vanadium is best avoided.
In the xe2x80x3680 patent and the related applications, a semi-insulating silicon carbide crystal is described that includes donor dopants, acceptor dopants and intrinsic point defects that produce deep level states. When the concentration of intrinsic point defects exceeds the difference between the concentration of donors and the concentration of acceptors, the states resulting from intrinsic point defects can provide semi-insulating characteristics in the functional absence of vanadium; i.e., including a minimal presence that is less than the presence that can affect the electronic properties of the crystal.
The requirements for and the advantages of semi-insulating substrates, their use in devices, particularly microwave devices, and the associated and particular requirements for silicon carbide semi-insulating substrates are set forth in detail in the xe2x80x3680 patent and related applications, and are generally well understood in the art from a background standpoint. Thus, they will not be repeated in detail herein. For reference purposes, a relevant discussion is set forth in the xe2x80x3680 patent at column 1, line 14 through column 3, line 33, which is incorporated entirely herein by reference.
To this discussion it should be added, however, that the ever-increasing demand for wireless communication services, including high bandwidth delivery of Internet access and related services, drives a corresponding demand for devices and circuits that can support such delivery, which in turn calls for materialssuch as semi-insulating silicon carbidefrom which devices having the required capabilities can be manufactured.
Accordingly, the xe2x80x3680 patent explains that superior microwave performance can be achieved by the fabrication of silicon carbide field effect transistors (FETs) and related devices on high purity, vanadium-free semi-insulating monocrystalline silicon carbide substrates. As set forth in the xe2x80x3680 patent, the substrates derive their semi-insulating properties from the presence of intrinsic (point defect related) deep electronic states lying near the middle of the silicon carbide bandgap. The intrinsic deep states generally arise during growth of a crystal boule at high temperatures from which substrate wafers are cut in a manner generally well understood in this art.
In devices that incorporate these substrates, and in order to provide the appropriate low-loss RF performance, the substrate must act as a low-loss dielectric medium by continuously maintaining its semi-insulating characteristics. In turn, the ability to maintain semi-insulating behavior is dependent upon the total number of intrinsic deep states in the substrate. In current practice, if the density of the intrinsic deep levels is not sufficiently high it has been observed in practice that the semi-insulating characteristics of the substrate can become reduced or functionally eliminated when subsequent steps are carried out on or using a semi-insulating silicon carbide wafer. Such steps include the growth of epitaxial layers at temperatures of about (for illustrative purposes) 1400xc2x0 or above on the semi-insulating silicon carbide wafer. movemoveThis in turn reduces the number of useful devices that can be formed on or incorporating the wafers.
Although the inventors do not wish to be bound by any particular theory, it appears that when semi-insulating silicon carbide substrate wafers of this type are subjected to process steps at temperatures within certain ranges, the subsequent processing can act as an anneal that reduces the number of point defects. This can be thought of in the positive sense that a higher quality crystal is created, but it is disadvantageous when the number of intrinsic point defects is the basis for the semi-insulating character of the substrate wafer.
Stated differently, if kept within a particular temperature range for a sufficient time, the crystal equilibrium or near-equilibrium will shift to one in which the number of point defects is reduced; i.e., the crystal becomes more ordered (fewer point defects) at lower temperatures than it was at higher temperatures, in a manner expected in accordance with well-understood thermodynamic principles.
Accordingly, a need exists for silicon carbide substrate wafers that incorporate the advantages set forth in the xe2x80x3680 patent, and that can maintain these advantages during subsequent manufacture of devices and circuits on or incorporating the semi-insulating silicon carbide substrate wafers.
Accordingly, it is an object of the invention to produce semi-insulating resistivity in high purity silicon carbide crystals, and to do so in a manner that results in a silicon carbide crystals that maintain their semi-insulating characteristics during and after subsequent device processing and manufacture.
The invention meets this object with a method of producing high quality semi-insulating silicon carbide crystals in the absence of relevant amounts of deep level trapping elements. The method comprises heating a silicon carbide crystal to a temperature above the temperatures required for CVD growth of silicon carbide from source gases, but less than the temperatures at which disadvantageously high rates of silicon carbide sublimation occur under the ambient conditions to thereby thermodynamically increase the concentration (i.e., number per unit volume) of point defects and resulting states in the crystal; and then cooling the heated crystal to approach room temperature at a sufficiently rapid rate to minimize the time spent in the temperature range in which the defects are sufficiently mobile to disappear or be re-annealed into the crystal to thereby produce a silicon carbide crystal with a concentration of point defect states that is greater than the concentration of point defect states in an otherwise identically grown silicon carbide crystal that has not been heated and cooled in this manner.
In another aspect, the invention is the semi-insulating silicon carbide crystal made by the method of the invention.
In yet another aspect, the invention is a method of producing semiconductor device precursors on semi-insulating substrates. In this aspect the invention comprises heating a silicon carbide substrate wafer to a temperature of at least about 2000xc2x0 C., then cooling the heated wafer to approach room temperature at a rate of at least about 30xc2x0 C. per minute, and then depositing an epitaxial layer of a semiconductor material on the substrate wafer.
The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the followed detailed description taken in conjunction with the accompanying drawings in which: